Optical transmission system for substantially equalizing the output signal level among frequency bands and substantially lowering the signal-to-noise ratio

ABSTRACT

In the WDM systems, the OSNR and the signal loss among the optical signals are substantially minimized at the receiving terminal to combat the SRS effects. An equal amount of the signal loss is expected for every span in the transmission path so that the optical amplifier gain tilt is not affected among a number of wavelength frequencies in the optical signal. This is accomplished by controlling the amplification process according to a feedback from the monitoring units for monitoring the optical signal.

FIELD OF THE INVENTION

[0001] The current invention is generally related to an opticaltransmission system, and more particularly related to substantiallyequalizing the output signal level among frequency bands andsubstantially lowering the signal-to-noise ratio.

BACKGROUND OF THE INVENTION

[0002] Wavelength Division Multiplexing (WDM) transmission systems havebeen used as a means to increase the transmission capacity. WDM allows asingle optical fiber to send a plurality of optical signals each with avarying wavelength. Optical fiber amplifiers such as an Erbium DopedFiber Amplifier (EDFA) simultaneously amplify a wide range ofwavelengths. Due to the above simultaneous amplification, thecombination of WDM and optical amplifiers enables economicaltransmission in a large capacity over a long distance with relativelysimple structures.

[0003] To cover a long distance, optical amplifiers are used insuccession for a series of spans or portions. Because the spans are notnecessarily equal, the actual placement of the optical amplifiers isalso not equidistant. Consequently, the amount of transmission lossvaries for each span. For the above varying transmission loss, the inputoptical level to an optical amplifier is not constant for each opticalamplifier, and the gain tilt occur among optical signals with differentwavelengths.

[0004] The above gain tilt is accumulated over multiple opticalamplifiers that are connected to a single optical fiber. At a finalreceiving unit, the gain tilt appears as a significant problem over along distance. Since the gain tilt may exceed a predetermined receptiondynamic range of a receiving unit, acceptable reception may not bepossible. The optical signal levels during the transmission also varyamong the wavelengths, and each optical signal receives a varyingnon-liner effect. Because of the above reasons, the optical signalwaveform is affected for each wavelength.

[0005] The WDM optical gain transmission generally requires an opticalsignal at a high input level to be entered into an optical fiber inorder to implement a long distance transmission system. It is concernedthat the above described non-linear degrading effect on transmissionwould be further exaggerated due to the high optical input signal level.One of the non-linear effects is Stimulated Raman Scattering (SRS),which causes excessive loss or gain in optical signals. SRS is anon-linear optical process where a portion of the optical fiber inputsignal acts as stimulating light and energy moves from high-frequencysignals to low-frequency signals by interacting with the low-frequencysignals in the optical fibers. Although SRS occurs in all opticalfibers, the effect depends upon the type of fibers and the frequencydifference between the optical signals that are involved in energytransfer.

[0006] There is a number of factors for the amount of the abovedescribed energy transfer. The energy transfer is proportional to thesum of the output optical strength. The more wavelengths there are andthe wider the wavelength range is in the WDM apparatus, the larger theamount of the energy transfers. As a result, the SRS effect is moretypically seen. Further, the longer the transmission path is, the moreapparent the SRS effect becomes. Under the influence of the SRS effect,the WDM transmission experiences a varying Optical Signal to Noise Ratio(OSNR) among the optical signals with a different wavelength since thewavelength signal level discrepancy occurs during transmission and theoptical input level varies for each optical amplifier. Similar to theinter-wavelength gain tilt, the optical output strength also varies foreach wavelength during transmission. As a result of the varying outputstrength, the reception waveform also experiences wavelength-dependentwaveform distortions and transmission errors due to self phasemodulation and wavelength dispersion as well as frequency chirping.

[0007] Prior art WDM systems had operated with a relatively small numberof wavelengths and a relatively narrow range of frequencies in opticalsignals. For the above reasons, the effect of the inter-wavelength gaintilt and the SRS-induced signal level discrepancy had not beensignificant. Contrarily, as the communication traffic has increased inthe recent years, the WDM systems have been demanded to cover a longerdistance, to accommodate a larger number of frequency signals and towiden a range of the frequency. As a result of the added requirements,the effect of the inter-wavelength gain tilt and the SRS-induced signallevel discrepancy can be no longer ignored. It has been reported inacademics that transmission becomes disabled due to the above describedincreased SRS effects when the initial design capacity for thewavelengths is reached in the WDM systems.

[0008] To deal with the above described undesirable effects in the WDMsystems, prior art has considered the discrepancy in gain tilt inoptical amplifiers and the wavelength-dependent loss in the transmissionpath. For example, Japanese Patent Publication Hei 8-223136 discloses amethod of minimizing the gain tilt in the optical amplifiers to obtainthe minimal OSNR. Another example is Japanese Patent Publication Hei11-55182, which discloses a method of minimizing the optical signallevel discrepancy and the OSNR at a receiving end. The above prior artmethods are useful for the situations where the optical signal lossamount is equal among the spans in the transmission path. Alternatively,the above prior art is also useful if the optical signal loss and theoptical amplifier gain tilt are not affected by a number of wavelengthfrequencies in the optical signal to be inputted in an opticalamplifier. Unfortunately, the above described prior art technologies donot account for the situations where the optical signal loss amount isnot equal among the spans in the transmission path. The above prior arttechnologies also fail consider that the optical signal output strengthis different among a number of wavelength frequencies or ranges offrequencies in the optical signal.

[0009] Japanese Patent Publication 2000-183818 discloses the method ofadjusting optical signal strength at a transmission side for eachwavelength by pre-emphasis so as to stabilize the OSNR at a receivingside. However, the above method depends upon a range and a number offrequencies, and the discrepancy or tilt in signal level among theoptical signals may exceed the pre-emphasis guaranty.

[0010] For the above described reasons, it is desirable to guarantee theOSNR among the optical signals at the receiving terminal in the WDMsystems. In other words, it is desirable to guarantee an equal opticalsignal loss amount among the spans in the transmission path so that theoptical amplifier gain tilt is not affected among a number of wavelengthfrequencies in the optical signal. Furthermore, it is also desirable toguarantee the signal level among frequencies due to SRS during theoptical fiber transmission.

SUMMARY OF THE INVENTION

[0011] In order to solve the above and other problems, according to afirst aspect of the current invention, a method of controlling opticalsignal during transmission, including the steps of: transmitting a wavedivision multiplexed optical signal having a predetermined set of rangesof wavelengths; amplifying the wave division multiplexed optical signalto generate an amplified wave division multiplexed optical signal;monitoring a total optical strength level of one of the ranges of theamplified wave division multiplexed optical signal; monitoring a probeoptical strength level of one of the wavelengths of the amplified wavedivision multiplexed optical signal; and adjusting the amplificationbased upon the total optical strength level and the probe opticalstrength level so as to substantially reduce a gain tilt and an opticalsignal-to-noise ratio in the amplified wave division multiplexed opticalsignal.

[0012] According to a second aspect of the current invention, a systemfor controlling optical signal during transmission, including: a firstand second optical fibers for transmitting a wave division multiplexedoptical signal having a predetermined set of ranges of wavelengths; anamplifier connected to the first optical fiber for amplifying the wavedivision multiplexed optical signal according to a predeterminedamplification characteristic to generate an amplified wave divisionmultiplexed optical signal, the amplifier outputting the amplified wavedivision multiplexed optical signal to the second optical fiber; a firstmonitor connected to the second optical fiber for monitoring a totaloptical strength level of one of the ranges of the amplified wavedivision multiplexed optical signal; a second monitor connected to thesecond optical fiber for monitoring a probe optical strength level ofone of the wavelengths of the amplified wave division multiplexedoptical signal; and an adjustment unit connected to the amplifier, thefirst monitor and the second monitor for adjusting the amplificationcharacteristic based upon the total optical strength level and the probeoptical strength level so as to substantially reduce a gain tilt and anoptical signal-to-noise ratio in the amplified wave division multiplexedoptical signal.

[0013] These and various other advantages and features of novelty whichcharacterize the invention are pointed out with particularity in theclaims annexed hereto and forming a part hereof. However, for a betterunderstanding of the invention, its advantages, and the objects obtainedby its use, reference should be made to the drawings which form afurther part hereof, and to the accompanying descriptive matter, inwhich there is illustrated and described a preferred embodiment of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a diagram illustrating a Wavelength DivisionMultiplexing (WDM) transmission system in which optical amplifiersgenerate a varying gain tilt among wavelengths due to a different amountof transmission loss per span.

[0015]FIG. 2A is a graph illustrating a predetermined input level acrossthe above wavelength ranges.

[0016]FIG. 2B is a graph illustrating that the output optical signallevel is higher at the long wavelength ranges than that at the shortwave length ranges.

[0017]FIG. 3 is a diagram illustrating one preferred embodiment of theWDM systems for transmitting optical signals according to the currentinvention.

[0018]FIG. 4 is a diagram illustrating a first preferred embodiment ofthe optical amplifier according to the current invention.

[0019]FIG. 5 is a graph illustrating an exemplary spectrum of an opticalsignal for determining a total optical strength level, an averageoptical strength level and a gain tilt.

[0020]FIG. 6 is a diagram illustrating one exemplary implementation ofthe above described output level control in the preferred embodimentaccording to the current invention.

[0021]FIG. 7 is a diagram illustrating another exemplary implementationof the above described output level control in the preferred embodimentaccording to the current invention.

[0022]FIG. 8 is a diagram illustrating yet another exemplaryimplementation of the above described gain tilt control in the preferredembodiment according to the current invention.

[0023]FIG. 9 is an exemplary graph illustrating various degrees of gaintilt.

[0024]FIG. 10 is a diagram illustrating one example of adjusted gaintilt during the multi-stage transmission.

[0025]FIG. 11 is a diagram illustrating a total optical signal includinga signal strength portion at the top and a noise strength portion belowthe signal strength.

[0026]FIG. 12 is a diagram illustrating another example of adjusted gaintilt during the multi-stage transmission.

[0027]FIG. 13 is a diagram illustrating a second preferred embodiment ofthe optical amplifier according to the current invention.

[0028]FIG. 14 is a diagram illustrating that the second preferredembodiment further includes a discrepancy table.

[0029]FIG. 15 is a diagram illustrating a third preferred embodiment ofthe optical amplifier according to the current invention.

[0030]FIGS. 16A and 16B are diagrams illustrating the effect of thecontrol as expressed in Equation 3.

[0031]FIG. 17 is a diagram illustrating the effects of the SRS atvarious points in the transmission and the processing results by thethird preferred embodiment of the optical amplifier according to thecurrent invention.

[0032]FIGS. 18A and 18B are a flow chart illustrating steps involved ina preferred process of adjusting the amplification of a wavelengthdivision multiplexed (WDM) optical signal according to the currentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0033] Referring now to the drawings, wherein like reference numeralsdesignate corresponding structures throughout the views, and referringin particular to FIG. 1, a diagram illustrates that optical amplifiersgenerate a varying gain tilt among wavelengths due to a different amountof transmission loss per span in the Wavelength Division Multiplexing(WDM) transmission system. An optical transmission device 101 transmitswavelength division multiplexed signals towards a preamplifier 103. Thepreamplifier 103 amplifies the wavelength division multiplexed signalsand transmits the amplified division multiplexed signals in an opticalfiber 400 a towards a line amplifier 203 a. During the transmission viathe optical fiber 400 a, the wavelength division multiplexed signalsexperience signal loss. The line amplifier 203 a amplifies thewavelength division multiplexed signals after the signal loss andfurther transmits the amplified signal via an optical fiber 400 b. Theabove described process is repeated as illustrated by additional lineamplifiers 203 b and 203 c as the division multiplexed signals travelthrough an optical fiber portion 400 c and 400 d. A different number ofcircles at the optical fiber portions 400 a, 400 b, 400 c and 400 ddenoted a varying distance for a corresponding span. Before the divisionmultiplexed signals reach a reception unit 307, a post amplifier 303amplifies the division multiplexed signals.

[0034] Still referring to FIG. 1, at each of the amplifiers 103, 203 a,203 b, 203 c and 303, a spectrum of the division multiplexed signals isillustrated below the corresponding one of the amplifier symbols. Thespectrum is illustrated to express the wavelength in the x axis and theoptical signal strength in the y axis. As shown in the diagram, sincethe distance between amplifiers or the span in the long distancemulti-stage transmission path varies, the signal loss also varies foreach span. In general, to control the optical strength in the outputsignal from the optical amplifiers, the optical strength of the inputsignal to the optical amplifiers is often limited to a predeterminedvalue. For example, when the optical input signal is maintained at −5.5dBm, although an average output light strength level is constant foreach span, the gain tilt still exists among optical signals at differentwavelengths.

[0035] Now referring to FIG. 2, graphs illustrate the effect of SRS onthe signal level of various wavelength-based optical signals. Ingeneral, an x-axis represents wavelength while a y-axis represents anoutput level or optical strength of the wavelength division multiplexedoptical signals across the wavelength spectrum. For example, wavelengthranges B, R, L1 and L2 respectively range from 1530 nm to 1545 nm, from1545 to 1560, from 1560 nm to 1575 nm and from 1575 nm to 1590 nm. Inparticular, referring to FIG. 2A, a predetermined input level ismaintained across the above wavelength ranges B, R, L1 and L2. Despitethe same input optical signal strength level across the wavelengthranges B, R, L1 and L2, FIG. 2B illustrates that the output opticalsignal level is higher at the long wavelength ranges L1 and L2 than thatat the short wave length ranges B and R. In fact, the output opticalstrength sequentially increases from the shortest range B to the longestrange L2. The above described gain tilt occurs due to the energytransfer from the short wavelength optical signals to the longwavelength optical signals.

[0036]FIG. 3 is a diagram illustrating one preferred embodiment of theWDM systems for transmitting optical signals according to the currentinvention. In general, the preferred embodiment includes a transmittingterminal device 100, an intermediate station or booster device 200, areceiving terminal device 300, optical fibers 400(1) through 400(k−1)that connect the above devices 100, 200 and 300 as well as a monitorcontrol device 500. Since the monitor control device 500 controls andmonitors the entire system, the monitor control device 500 is providedseparately from other devices. In the preferred embodiment, fourseparate wavelength bands B, R, L1 and L2 respectively cover a rangefrom 1530 nm to 1545 nm, from 1545 to 1560, from 1560 nm to 1575 nm andfrom 1575 nm to 1590 nm. For each of the ranges B, R, L1 and L2, aseparate amplifier is provided for amplifying the optical signals in thetransmitting terminal device 100, the intermediate station or boosterdevice 200 and the receiving terminal device 300. For each of the abovedescribed amplifiers, a predetermined number N of optical signals ismultiplexed with a varying wavelength.

[0037] Still referring to FIG. 3, each of the above devices 100, 200 and300 is described in the following. The transmitting terminal device 100further includes a set of optical transmitters for each range of thewavelength optical signals. For the B band, the optical transmitters101(1) through 101(n) transmit a predetermined n number of opticalsignals with a varying wavelength. A corresponding multiplexer 102(1)wavelength-division multiplexes the transmitted optical signals from theoptical transmitters 101(1) through 101(n). Subsequently, acorresponding amplifier 103(1) amplifies the wavelength-divisionmultiplexed optical signal from the multiplexer 102(1). Similarly, forthe R band, the optical transmitters 101(n+1) through 101(2n) transmitan additional n number of optical signals with a varying wavelength. Acorresponding multiplexer 102(2) wavelength-division multiplexes thetransmitted optical signals from the optical transmitters 101(n+1)through 101(2n). Subsequently, a corresponding amplifier 103(2)amplifies the wavelength-division multiplexed optical signal from themultiplexer 102(2). For the L1 band, the optical transmitters 101(2n+1)through 101(3n) transmit an additional n number of optical signals witha varying wavelength. A corresponding multiplexer 102(3)wavelength-division multiplexes the transmitted optical signals from theoptical transmitters 101(n+1) through 101(3n). Subsequently, acorresponding amplifier 103(3) amplifies the wavelength-divisionmultiplexed optical signal from the multiplexer 102(3). For the L2 band,the optical transmitters 101(3n+1) through 101(4n) transmit anadditional n number of optical signals with a varying wavelength. Acorresponding multiplexer 102(4) wavelength-division multiplexes thetransmitted optical signals from the optical transmitters 101(3n+1)through 101(4n). Subsequently, a corresponding amplifier 103(4)amplifies the wavelength-division multiplexed optical signal from themultiplexer 102(4). Lastly, a second multiplexer 104 furtherwavelength-division multiplexes the amplified wavelength-divisionmultiplexed optical signals from the amplifiers 101(1) through 101(4) totransmit the multiplexed optical signal to the booster device 200 viathe optical fiber 400(1).

[0038] The booster device 200 further includes a wavelength-divisiondemultiplexer 205, a set of optical amplifiers 203(1) through 203(4) anda wavelength-division multiplexer 204. The wavelength-divisiondemultiplexer 205 demultiplexes the wavelength-division multiplexedoptical signals from the transmitting device 100 into the fourwavelength ranges B, R, L1 and L2. For each of the demultiplexed bandsB, R, L1 and L2, line amplifiers 203(1) through 203(4) respectivelyamplify the optical signals to compensate the corresponding transmissionloss. The optically amplified signals are again wavelength-divisionmultiplexed in a multiplexed 204. The multiplexed signals aretransmitted via optical fibers 400(2). Although FIG. 2 abbreviates therepetition of the above described booster devices by the dotted line, aseries of the booster devices and corresponding optical fibers through afinal optical fiber portion 400(k−1) are placed depending upon atransmission distance to the receiving device 300.

[0039] Still referring to FIG. 3, the reception device 300 furtherincludes a demultiplexer 305 to demultiplex the wavelength-divisionmultiplexed input signal from the optical fiber 400(k−1) into theoriginal four bands B, R, L1 and L2. Post amplifiers 303(1) through303(4) respectively amplify the demultiplexed optical signals for thefour bands B, R, L1 and L2. A set of additional demultiplexers 305(1)through 305(4) further demultiplexes the amplified initiallydemultiplexed band signals. For the B band, the demultiplexer 306(1)demultiplexes the optical signal to generate the originallycorresponding optical signals for optical receivers 307(1) through307(n). Similarly, for the R band, the demultiplexer 306(2)demultiplexes the optical signal to generate the originallycorresponding optical signals for optical receivers 307(n+1) through307(2n). For the L1 band, the demultiplexer 306(3) demultiplexes theoptical signal to generate the originally corresponding optical signalsfor optical receivers 307(2n+1) through 307(3n). Lastly, for the L2band, the demultiplexer 306(4) demultiplexes the optical signal togenerate the originally corresponding optical signals for opticalreceivers 307(3n+1) through 307(4n).

[0040] Now referring to FIG. 4, a diagram illustrates a first preferredembodiment of the optical amplifier according to the current invention.The diagram illustrates that an optical amplifier 700A monitors a totaloutput optical strength Pn_total 715 and an output probe opticalstrength Pn_prob 712 for an optical wavelength signal that is includedin the WDM optical signal. Alternatively, the output probe opticalstrength Pn_prob 712 is in a predetermined amplification range. Ingeneral, the optical amplifier 700A simultaneously controls the totaloutput optical strength 715 in the amplification range and the gaintilt. In particular, an amplifier 701 amplifies an optical input signal705 and outputs an optical output signal 706. An output level controlunit 703 controls the output strength of the amplifier 701 while a gaintilt control unit 710 controls the gain tilt of the amplifier 701. Anoutput level setting unit 704 stores an external signal 707 forspecifying a target optical output signal strength level. A gain tiltsetting unit 713 stores an external signal 708 for specifying the gaintilt. The external signal 708 is an average optical strength that isdetermined by dividing the total output optical strength Pn_total 715 bya number of wavelengths that is amplified by the amplifier 701. A narrowoptical range filter 711 filters out a certain wavelength probe signalfrom the amplified output signal from the amplifier 701 and inputs thefiltered wavelength probe signal to an optical strength monitor B 716.The optical strength monitor B 716 converts the optical wavelength probesignal to an electrical signal Pn_prob 712.

[0041] Still referring to FIG. 4, similarly, an optical strength monitorA 717 receives the amplified output signal from the amplifier 701 andconverts the optical signal to an electrical signal Pn_total 715. Acomparator B 709 compares the gain tilt value from the gain tilt settingunit 713 and the electrical signal Pn_prob 712 from the optical strengthmonitor B 716. The output of the comparator B 709 is inputted into again tilt control unit 710. Based upon the comparison result from thecomparator B 709, the gain tilt control unit 710 automatically controlsthe amplifier 701 so as to generate the optical probe signal Pn_prob 712at a level that is stored in the gain tilt setting unit 713. If theabove comparison result from the comparator B 709 is negative, thecontrol performs to negate the negative in the output. On the otherhand, a comparator A compares the optical output strength from thetarget output level setting unit 704 and the electrical signal Pn_total715 from the optical strength monitor A 717. The output of thecomparator A 702 is inputted into an output level control unit 703.Based upon the comparison result from the comparator A 702, the outputlevel control unit 703 automatically controls the amplifier 701 so as togenerate the optical output signal 706 at a level that is stored in theoutput level setting unit 707. If the above comparison result from thecomparator A 702 is negative, the control performs to negate thenegative in the output.

[0042]FIG. 5 is a graph illustrating an exemplary spectrum of an opticalsignal for determining a total optical strength level, an averageoptical strength level and a gain tilt. The optical signal includes fourbands or frequency ranges B, R, L1 and L2. For each of the bands B, R,L1 and L2, the total optical strength level includes a B-band totaloptical strength level 810, a R-band total optical strength level 811, aL1-band total optical strength level 812 and a L2-band total opticalstrength level 813. For each band, the total optical strength level isdivided by a number of wavelengths involved in the corresponding band todetermine the average optical strength level. In other words, thefollowing Equation(1) holds:

Probe Light=(average optical strength)=(total optical strength )/(anumber of wavelengths)

[0043] The average optical strength level includes a B-band averageoptical strength level 806, a R-band average optical strength level 807,a L1-band average optical strength level 808 and a L2-band averageoptical strength level 809. Furthermore, for each band, an optical probesignal is determined and includes a B-band optical probe signal 814, aR-band optical probe signal 815, a L1-band optical probe signal 816 anda L2-band optical probe signal 817.

[0044] Still referring to FIG. 5, certain parameters are determinedbased upon a comparison of the above described values. By comparing theaverage optical strength level and the optical probe signal for eachband, a gain tilt is determined between the bands at the output side.Between the B-band and the R-band, a B-R inter band gain tilt isdetermined to be a gain tilt 802. Similarly, between the R-band and theL1-band, a R-L1 inter band gain tilt is determined to be a gain tilt804. Between the L1-band and the L2-band, a L1-L2 inter band gain tiltis determined to be a gain tilt 805. By comparing the average opticalstrength level and the optical probe signal for each band, a gain tiltis determined within each of the bands. An inter-band gain tilt 818 isdetermined for the B-band while an inter-band gain tilt 819 isdetermined for the R-band. To clearly define the R inter-band tilt gain819, a portion of the diagram that corresponds to the R-band isenlarged. The R inter-band tilt gain 819 is a difference between theR-band optical probe signal 815 and the R-band average optical strengthlevel 807. Similarly, an inter-band gain tilt 820 is determined for theL1-band while an inter-band gain tilt 821 is determined for the L2-band.The above described parameters are simultaneously determined.

[0045] Now referring back to FIG. 4, a diagram illustrates a preferredembodiment of the optical amplifier unit according to the currentinvention. The preferred embodiment monitors determines the output level706 at a desired level for optical signals across the frequency band. Asdescribed above, based upon the information in the output level settingunit 704, the output level control unit 703 controls the amplifier 701to generate the output optical signal 706 at a desired level. The gaintilt setting unit 713 stores a gain tilt setting value that is comparedto the output signal from the optical strength monitor B 716. In thepreferred embodiment, the stored gain tilt value is an average opticalstrength value so that the comparison allows the gain tilt control unit710 to control the amplifier 701 to generate the output optical signal706 at a constant strength across the wavelength.

[0046] To control the above described parameters such as the outputlevel and the gain tilt, the optical output signal 706 from the opticalamplifier 700A is adjusted in the preferred embodiment according to thecurrent invention. Referring to FIG. 6, a diagram illustrates oneexemplary implementation of the above described output level control inthe preferred embodiment according to the current invention. Theamplifier 701A further includes an EDF 901 and an excitation LD unit902. The output level control unit 703 inputs a signal indicative of thedesired output level into the excitation LD unit 902, which in turnexcites the EDF 901 for amplifying the input optical signal 705 to theoutput optical signal 706. Alternatively, referring to FIG. 7, a diagramillustrates another exemplary implementation of the above describedoutput level control in the preferred embodiment according to thecurrent invention. The amplifier 701B further includes an EDF 901, anexcitation LD unit 902 and a variable attenuator 1001. The output levelcontrol unit 703 inputs a signal indicative of the desired output levelinto the attenuator 1001, which in turn attenuates an optical outputsignal from the EDF 901 after amplifying the input optical signal 705via an excitation LD unit 902. Lastly, referring to FIG. 8, a diagramillustrates yet another exemplary implementation of the above describedgain tilt control in the preferred embodiment according to the currentinvention. The amplifier 701C further includes an EDF 901, an excitationLD unit 902 and a variable gain tilt adjustment unit 1002. The gain tiltcontrol unit 710 inputs a signal indicative of the desired gain tiltlevel into the gain tilt adjustment unit 1002, which in turn adjusts anoptical output signal from the EDF 901 after amplifying the inputoptical signal 705 via an excitation LD unit 902. The variable gain tiltadjustment unit 1002 adjusts the output strength among optical signalswith a variable wavelength.

[0047] The gain tilt control is illustrated in an exemplary graph inFIG. 9. The x axis of the graph represents a range of wavelengths whilethe y axis of the graph represents an output level. A line 1601represents a range of predetermined optical signals within apredetermined range of frequencies, and the line 1601 indicates that theoutput optical level is uniform within the frequency range. On the otherhand, lines 1602 and 1603 indicate a situation where the output signallevel at the high frequency side is lower than that at the low frequencyside. Lines 1604 and 1605 indicate a situation where the output signallevel at the high frequency side is higher than that at the lowfrequency side.

[0048] Having explained one preferred embodiment of the gain tiltadjustment in the WDM transmission system, the adjusted gain tilt duringthe multi-stage transmission is further illustrated in FIG. 10. Asdescribed before, the transmission loss varies depending upon thedistance of each span. FIG. 10 is a diagram illustrating optical signalsat various stages of the transmission at the preamplifiers 103(1)through 103(4), the line amplifiers 203(1) through 203(4) and the postamplifiers 303(1) through 303(4) as shown in FIG. 3. The diagramillustrates the situation where the input optical signals have arelatively narrow frequency range and a number of the optical signal ata varying wavelength is also relatively small in the frequency range.Under the above described circumstances, the effects of SRS areinsignificant and are ignored. To interpret the optical signals at thevarious transmission stages as shown in FIG. 10, a total optical signalis represented by the sum of a signal strength portion at the top and anoise strength portion below the signal strength portion as shown inFIG. 11.

[0049] Still referring to FIG. 10, the diagram further illustrates theeffects of the SRS at various points in the transmission and theprocessing results by the first preferred embodiment of the opticalamplifier according to the current invention. The various points includethe preamplifier, the line amplifier and the post amplifier as shown inFIG. 3.

[0050] The exemplary signal includes four bands as illustrated in FIG. 2and is transmitted in the above transmission path. In the followingdescriptions of FIG. 10, references are made to the elements in FIGS. 2and 3. The WDM optical signal loses some optical strength after beingtransmitted through the optical fiber 400(1). As shown in the secondcolumn in FIG. 10, the optical signal generally has a lower energy leveland evenly across the entire spectrum at the points 220(1) through220(4) prior to amplification. The line amplifiers 203(1) through 203(4)amplify the respective weakened optical signal to a predetermined levelwith a uniform gain as shown in the third column in FIG. 10. Theamplified signals in the third column are taken at points 230(1) through230(4) subsequent to amplification by the line amplifiers 203(1) through203(4). The above described amplification is repeated for each spanuntil the signal is transmitted to the receiving unit 300 so that theoutput from the post amplifiers 303(1) through 303(4) are also at apredetermined level with a uniform gain. The amplified signals in thefourth column are taken at points 330(1) through 330(4) subsequent toamplification by the post amplifiers 303(1) through 303(4).Alternatively, the above described amplification is performed on certainspans until the signal is transmitted to the receiving unit.

[0051] The optical signal to noise ratio (SNR) in the optical signalsare somewhat wavelength dependent at the points 230(1) through 230(4) inthe output from the line amplifiers 203(1) through 203(4) and the points330(1) through 330(4) in the output from the post amplifiers 303(1)through 303(4). In other words, the optical signals at the points 230(1)through 230(4) have a higher noise ratio in high frequencies while theoptical signals at the points 330(1) through 330(4) have a higher noiseratio in low frequencies. As described with respect to FIG. 1 the noiseratio depends upon the transmission distance of the optical fibers 400,which results in a different input level to a corresponding opticalamplifiers 203(1) through 203(4) and 303(1) through 303(4). Thus, eventhough the initial optical signals have the equal optical strength, anincreasing or decreasing gain tilt results based upon the above factor.Even though the gain tilt compensation is performed, the effects of theabove described causes for the undesirable gain tilt may not becompletely eliminated. On the other hand, when the spans are short andor a number of wavelength division multiplexed frequencies is relativelysmall, the SRS effect is minimal. As the result, the gain tilt isrelatively small and the optical SNR among the bands is controlled to anacceptable level.

[0052] Now referring to FIG. 12, a diagram illustrates the exemplaryeffects of the SRS at various points in the transmission and theprocessing results by the first preferred embodiment of the opticalamplifier according to the current invention. The exemplary signalincludes four bands as illustrated in FIG. 2 and is transmitted in theabove transmission path. The optical signals are illustrated at variousstages of the transmission at the preamplifiers 103(1) through 103(4),the line amplifiers 203(1) through 203(4) and the post amplifiers 303(1)through 303(4) as shown in FIG. 3. The diagram illustrates the situationwhere the input optical signals have a relatively wide frequency rangeand a number of the optical signal at a varying wavelength is alsorelatively large in the frequency range. Under the above describedcircumstances, the effects of SRS are substantially significant andcannot be ignored. To interpret the optical signals at the varioustransmission stages as shown in FIG. 12, a total optical signal isrepresented by the sum of a signal strength portion at the top and anoise strength portion below the signal strength portion as shown inFIG. 11.

[0053] Still referring to FIG. 12, the diagram further illustrates theeffects of the SRS at various points in the transmission and theprocessing results by the first preferred embodiment of the opticalamplifier according to the current invention. The various points includeones with respect to the preamplifier, the line amplifier and the postamplifier as shown in FIG. 3. The exemplary signal includes four bandsas illustrated in FIG. 2 and is transmitted in the above transmissionpath. In the following descriptions of FIG. 12, references are made tothe elements in FIGS. 2 and 3. The WDM optical signal loses some opticalstrength after being transmitted through the optical fiber 400(1). Asshown in the second column in FIG. 12, the optical signal generally hasa lower energy level but unevenly across the entire spectrum at thepoints 220(1) through 220(4) prior to amplification. Due to the SRSeffects, some gain tilt has occurred in the optical signals at thepoints 220(1) through 220(4). In general, the optical strength is higherat high frequencies than at lower frequencies. The line amplifiers203(1) through 203(4) amplify the respective weakened optical signal toa predetermined level with a uniform gain as shown in the third columnin FIG. 12. The amplified signals in the third column are taken atpoints 230(1) through 230(4) subsequent to amplification by the lineamplifiers 203(1) through 203(4). Although the output level and the gaintilt are controlled in the spectrums 230(1) through 230(4) in the outputform the line amplifiers 203(1) through 203(4), the optical SNR varieswithin a band and among the bands due to the interaction between thegain tilt caused by the SRS effects and that caused by the input levelto the optical amplifiers 203(1) through 203(4). In comparison to theshort wavelength bands such as the band B as shown in the spectrum230(1), the longer the wavelength is, the more optimal the optical SNRbecomes. In fact, the most optimal SNR is seen in the longest wavelengthband L2 as shown in the spectrum 230(4) in which the noise component isthe least. The above described amplification is repeated for each spanuntil the signal is transmitted to the receiving unit 300 so that theoutput from the post amplifiers 303(1) through 303(4) are also at apredetermined level with a uniform gain. The amplified signals in thefourth column are taken at points 330(1) through 330(4) subsequent toamplification by the post amplifiers 303(1) through 303(4).

[0054] The optical signal to noise ratio (SNR) in the optical signals iswavelength dependent due to the input level to the line amplifiers203(1) through 203(4) and the SRS effects in the optical fiber 400(1)during transmission. As shown in FIG. 12, the above wavelengthdependency exists not only within a band but also among the bands.Contrary to the above example, in some case, the shorter wavelength is,the more optimal the optical SNR becomes. The gain tilt discrepancy iscaused by the varying input level to the optical amplifier as a resultof a varying amount of transmission loss per span. Because of thecombined effects of the gain tilt discrepancy and the SRS effects duringthe transmission, the wavelength-dependency of the optical SNRultimately becomes significant. As illustrated in the fourth column ofthe diagram in FIG. 12, the optical SNR is becomes better as thewavelength also becomes longer with in a band or among the bands. Infact, in the longest wavelength band L2, the optical SNR becomes thebest as shown in the spectrum 330(4). The discrepancy in the optical SNRis substantial among the spectra 330(1) through 330(4). Under theconditions with the significant wavelength-dependency of the opticalSNR, it becomes difficult to correctly receive the entire spectrum ofthe WDM optical signal.

[0055] Now referring to FIG. 13, a diagram illustrates a secondpreferred embodiment of the optical amplifier according to the currentinvention. The diagram illustrates that an optical amplifier 700Bmonitors an input optical strength Pn_total′ 719 and an input probeoptical strength Pn_prob′ 718 for an optical wavelength signal that isincluded in a predetermined amplification range. The diagram alsoillustrates that the optical amplifier 700B monitors a total outputoptical strength 715 and an output probe optical strength 712 for anoptical wavelength signal that is included in a predeterminedamplification range. In general, the optical amplifier 700Bsimultaneously controls the total output optical strength 715 in theamplification range and the gain tilt. In particular, an amplifier 701amplifies an optical input signal 705 and outputs an optical outputsignal 706. An output level control unit 703 controls the outputstrength of the amplifier 701 while a gain tilt control unit 710controls the gain tilt of the amplifier 701. An output level settingunit 704 stores an external signal 707 for specifying a target opticaloutput signal strength level. A gain tilt setting unit 713 stores anexternal signal 708 for specifying the gain tilt. A narrow optical rangefilter 711 filters out a certain wavelength probe signal from theamplified output signal from the amplifier 701 and inputs the filteredwavelength probe signal to an optical strength monitor B 716. Theoptical strength monitor B 716 converts the optical wavelength probesignal to an electrical signal Pn_prob 712.

[0056] Similarly, an optical strength monitor A 717 receives theamplified output signal from the amplifier 701 and converts the opticalsignal to an electrical signal Pn_total 715. A comparator B compares thegain tilt from the gain tilt setting unit 713 and the electrical signalPn_prob 712 from the optical strength monitor B 716. The output of thecomparator B 709 is inputted into a gain tilt control unit 710. Basedupon the comparison result from the comparator B 709, the gain tiltcontrol unit 710 automatically controls the amplifier 701 so as togenerate the optical probe signal Pn-prob 712 at a level that is storedin the gain tilt setting unit 713. If the above comparison result fromthe comparator B 709 is negative, the control performs to negate thenegative in the output. On the other hand, a comparator A compares theoptical output strength from the target output level setting unit 704and the electrical signal Pn_total 715 from the optical strength monitorA 717. The output of the comparator A 702 is inputted into an outputlevel control unit 703. Based upon the comparison result from thecomparator A 702, the output level control unit 703 automaticallycontrols the amplifier 701 so as to generate the optical output signal706 at a level that is stored in the output level setting unit 707. Ifthe above comparison result from the comparator A 702 is negative, thecontrol performs to negate the negative in the output.

[0057] Still referring to FIG. 13, the second preferred embodiment ofthe optical amplifier according to the current invention furtherincludes an additional pair of monitoring units at the input side. Anarrow optical range filter 720 filters out a certain wavelength probesignal from the input signal 705 and inputs the filtered wavelengthprobe signal to an optical strength monitor C 721. The optical strengthmonitor C 721 converts the optical wavelength probe signal to theelectrical signal Pn_prob′ 718. Similarly, an optical strength monitor D722 receives the input signal 705 to the amplifier 701 and converts theoptical input signal 705 to an electrical signal Pn_total′ 719.

[0058] In addition, the second preferred embodiment further includes adiscrepancy table 1201 as shown in FIG. 14. The discrepancy table 1201is connected to the optical amplifier 700B via a first input line 708 tothe gain tilt setting unit 713 and a second input line 707 to the outputlevel setting unit 704. Based upon the stored data in the discrepancytable 1201, the output level setting unit 704 receives a desired outputlevel value to be stored. Similarly, the gain tilt setting unit 713receives a desired gain tilt value to be stored. The output level valuesand the gain tilt values are calculated in advance and stored in thediscrepancy table 1201 rather than being determined on the fly. Theabove values are determined in a manner that will be discussed later.

[0059] In the second preferred embodiment of the optical amplifieraccording to the current invention, an exemplary optical signal isessentially identical at the output side as illustrated in FIG. 5. Theoptical signal includes four bands or frequency ranges B, R, L1 and L2.For each of the bands B, R, L1 and L2, the total optical strength levelincludes a B-band total optical strength level 810, a R-band totaloptical strength level 811, a L1-band total optical strength level 812and a L2-band total optical strength level 813. The average opticalstrength level includes a B-band average optical strength level 806, aR-band average optical strength level 807, a L1-band average opticalstrength level 808 and a L2-band average optical strength level 809.Furthermore, for each band, an optical probe signal is determined andincludes a B-band optical probe signal 814, a R-band optical probesignal 815, a L1-band optical probe signal 816 and a L2-band opticalprobe signal 817. Certain parameters are determined based upon acomparison of the above described values. By comparing the averageoptical strength level and the optical probe signal for each band, again tilt is determined between the bands at the output side. Betweenthe B-band and the R-band, a B-R inter band gain tilt is determined tobe a gain tilt 802. Similarly, between the R-band and the L1-band, aR-L1inter band gain tilt is determined to be a gain tilt 804. Betweenthe L1-band and the L2-band, a L1-L2inter band gain tilt is determinedto be a gain tilt 805. By comparing the average optical strength leveland the optical probe signal for each band, a gain tilt is determinedwithin each of the bands. An inter-band gain tilt 818 is determined forthe B-band while an inter-band gain tilt 819 is determined for theR-band. To clearly define the R inter-band tilt gain 819, a portion ofthe diagram that corresponds to the R-band is enlarged. The R inter-bandtilt gain 819 is a difference between the R-band optical probe signal815 and the R-band average optical strength level 807. Similarly, aninter-band gain tilt 820 is determined for the L1-band while aninter-band gain tilt 821 is determined for the L2-band. The abovedescribed parameters are simultaneously determined.

[0060] The second preferred embodiment of the optical amplifier isfurther implemented according to the current invention. As shown in FIG.6, the amplifier 701A further includes an EDF 901 and an excitation LDunit 902. The output level control unit 703 inputs a signal indicativeof the desired output level into the excitation LD unit 902, which inturn excites the EDF 901 for amplifying the input optical signal 705 tothe output optical signal 706. Alternatively, as shown in FIG. 7, theamplifier 701B further includes an EDF 901, an excitation LD unit 902and a variable attenuator 1001. The output level control unit 703 inputsa signal indicative of the desired output level into the attenuator1001, which in turn attenuates an optical output signal from the EDF 901after amplifying the input optical signal 705 via an excitation LD unit902. Lastly, as shown in FIG. 8, yet another exemplary implementation ofthe amplifier 701C further includes an EDF 901, an excitation LD unit902 and a variable gain tilt adjustment unit 1002. The gain tilt controlunit 710 inputs a signal indicative of the desired gain tilt level intothe gain tilt adjustment unit 1002, which in turn adjusts an opticaloutput signal from the EDF 901 after amplifying the input optical signal705 via an excitation LD unit 902. The variable gain tilt adjustmentunit 1002 adjusts the output strength among optical signals with avariable wavelength.

[0061] Now referring to FIG. 15, a diagram illustrates a third preferredembodiment of the optical amplifier according to the current invention.The third preferred embodiment of the optical amplifier is used as thepreamplifier 103 or the line amplifier 203. In a first WDM system 1403,the third preferred embodiment of the optical amplifier 700C furtherincludes an amplifier 701, an optical monitor 723, an output levelsetting unit 704, a gain tilt setting unit 713 and a central processingunit (CPU) 1401. The first WDM system 1403 is connected to a second WDMsystem 1404 via optical fibers 400, 1402. The CPU 1401 receives a totalinput optical strength signal Pn_total′ 719 and an input optical probesignal Pn_prob′ 718 via the optical fiber 1402 from the second WDMsystem 1404. In the optical fibers 1402, the total input opticalstrength signal Pn_total′ 719 and the input optical probe signalPn_prob′ 718 are transmitted in the opposite direction towards the firstWDM system 1403 from the second WDM system 1404. The The total inputoptical strength signal Pn_total′ 719 and the input optical probe signalPn_prob′ 718 are transmitted on the optical service channel (OSC) in theoptical fibers 1402. CPU 1401 also receives a total output opticalstrength signal Pn_total 715 and an output optical probe signal Pn_prob712. Based upon the above signals, the CPU 1401 calculates a pair ofcontrol signals and respectively transmits an output control signal 707and a gain tilt control signal 708 to the output level setting unit 704and the gain tilt setting unit 713. The output level setting unit 704and the gain tilt setting unit 713 store the received signal.

[0062] Still referring to FIG. 15, the central processing unit 1401generates the above described control signals. To generate the controlsignals, the central processing unit 1401 determines that the totaloptical output level of the frequency bands in the optical fiber isconstant. That is, as shown in the following Equation(2):${{\sum\limits_{n = 1}^{N}\quad {Pn\_ tota}} + {Pn\_ tota}^{\prime}} = {constant}$

[0063] Furthermore, the determination is based upon the total outputstrength Pn_total (n=1,2, . . . N) of each band and the total inputoptical strength Pn_total (n=1,2, . . . N) for the corresponding opticalamplifiers. n=1,2, . . . N and n indicates a particular band from apredetermined set of ranges of frequencies for the optical signals. Inother words, the control is performed to hold the following Equations(3)true: $\begin{matrix}{{{P1\_ total} + {P1\_ total}^{\prime}} = {{P2\_ total} + {P2\_ total}^{\prime}}} \\{= {{P3\_ total} + {P3\_ total}^{\prime}}} \\{= \ldots} \\{= {{Pn\_ total} + {Pn\_ total}^{\prime}}}\end{matrix}$

[0064] where n=1, 2, . . . N and n indicates a particular band from apredetermined set of ranges of frequencies for the optical signals. Thesum of the total optical strength Pn_′ total at a transmission unitbefore transmission and the total optical strength Pn_total′ at areceiving unit after transmission is identical among the frequency bands(n=1,2, . . . n).

[0065] Now referring to FIGS. 16A and 16B, diagrams illustrate theeffect of the control as expressed in Equation (3). A left diagram inFIG. 16A illustrates an optical signal whose optical strength level iseven across the bands P1_total 715-11 through P4_total 715-41 beforetransmission from an amplifier on the transmission side. Duringtransmission, the optical signal is affected by the SRS in the opticalfiber.). A right diagram in FIG. 16A illustrates the optical signalwhose optical strength level is uneven across the bands P1_titlt 715-12through P4_titlt 715-42 after transmission to a receiving unit from theamplifier on the transmission side. To compensate the above gain tiltafter transmission, a left diagram in FIG. 16B illustrates an opticalsignal whose optical strength level is processed to be uneven across thebands P1_total 715-13 through P4_total 715-43 before transmission viaamplifier control. In other words, the optical strength portionsP1_titlt 715-12 through P4_titlt 715-42 are respectively subtracted fromthe original even optical signal across the bands P1_total 715-13through P4_total 715-43. As a result of the above pre-compensation, aright diagram in FIG. 16B illustrates the optical signal whose opticalstrength level is now even across the bands P1_total′ 715-14 throughP4_total′ 715-44 after transmission to a receiving unit from theamplifier on the transmission side. As described above, to compensatethe gain tilt in each band at the receiving unit due to the SRS effect,the same amount of the gain tilt in the opposite direction iscompensated at the transmission side in advance of transmission.

[0066] In addition, the gain tilt control unit uses the probe opticalinput Pn_prob and the total input optical strength Pn_total that is froman optical amplifier input portion that is located adjacent to theoptical amplifier to be controlled. The gain tilt control in the opticalamplifier is expressed by the following Equation(4):

(Pn _(—) total+Pn _(—) total′)/(a number of wavelengths)=Pn _(—) prob+Pn_(—) prob′

[0067] where n=1,2, . . . N and n indicates a particular band from apredetermined set of ranges of frequencies for the optical signals. Thefollowing Equations(5) and (6) express the control that is applied tothe post amplifiers depending upon necessity in order to flatten thegain tilt within a band or among the bands.

P1_(—) total=P2_(—) total=P3=P4_(—) total

Pn _(—) prob=Pn _(—) total/(a number of wavelengths)

[0068] where n=1,2, . . . N and n indicates a particular band from apredetermined range of frequencies for the optical signals.

[0069] Now referring to FIG. 17, a diagram illustrates the effects ofthe SRS at various points in the transmission and the processing resultsby the third preferred embodiment of the optical amplifier according tothe current invention. The various points include the preamplifier, theline amplifier and the post amplifier as shown in FIG. 3. The exemplarysignal includes four bands as illustrated in FIG. 2 and is transmittedin the above transmission path. In general, the output strength from theintermediate optical amplifiers is adjusted so that the optical signalto noise ratio (SNR) is substantially identical among the bands in theoutput from the last post amplifier. As the result of the abovecorrection, the optical SNR is guaranteed within the bands and betweenthe bands.

[0070]FIGS. 18A and 18B are a flow chart illustrating steps involved ina preferred process of adjusting the amplification of a wavelengthdivision multiplexed (WDM) optical signal according to the currentinvention. In general, the preferred process adjusts the amplificationof the WDM optical signal during a transmission over a predetermineddistance so that the transmitted signal experiences a minimal amount ofsignal loss as well as gain tilt. The preferred process is describedwith respects to the above described units of the first, second andthird preferred embodiments as respectively shown in FIGS. 4, 13 and 15.Referring to FIG. 18A, in a step S1, a WDM optical signal is inputtedinto the amplifier unit 700A, 700B or 700C. The optical strengthmonitors C 721 and D 722 respectively monitor a probe optical strengthlevel and a total optical strength level prior to amplification in astep S2. The amplifier 701 amplifies the WDM optical signal according toa specified output level and a gain tilt in step S3. While theamplification in the step S3 takes place, the optical strength monitorsA 717 and B 716 monitor the amplified WDM optical signal from theamplifier 701 in S4. The comparators A 702 and B 709 respectivelycompare the corresponding monitored optical signal to a relevantpredetermined value in a step S5. In particular, the comparator A 702compares the total optical strength to a predetermined output levelvalue while the comparator B 709 compares the probe optical strength toa predetermined gain tilt value. The comparison results are used toadjust the amplification characteristics of the amplifier 701 in a stepS6.

[0071] Now referring to FIG. 18B, additional steps are further performedin the preferred process. After the WDM optical signal is appropriatelyamplified, the amplified WDM optical signal is transmitted towards areceiving unit via an optical fiber in a step S7. The distance for thetransmission in the step S7 is a span. At the receiving unit, thetransmitted WDM optical signal is monitored in a step S8, and themonitored results are fed back to the amplifier unit 700C. Based uponthe above monitored results from the receiving unit in the step S8 andthe monitored results from the step S4, the CPU 1401 determines theadjustment characteristics for the amplifier 701 in a step S9. Finally,based upon the above determined characteristic from the step S9, theamplifier 701 is adjusted in a step S10.

[0072] In the first, second and third preferred embodiment, a separateoptical amplifier is provided for each of the bands in the WDMtransmission system. In an alternative embodiment, a series of commonoptical amplifiers is used for all of the bands, and every one orcertain ones of the optical amplifiers is adjusted for gain tilt so asto substantially reduce the OSNR.

[0073] It is to be understood, however, that even though numerouscharacteristics and advantages of the present invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, the disclosure is illustrativeonly, and that although changes may be made in detail, especially inmatters of shape, size and arrangement of parts, as well asimplementation in software, hardware, or a combination of both, thechanges are within the principles of the invention to the full extentindicated by the broad general meaning of the terms in which theappended claims are expressed.

What is claimed is:
 1. A method of controlling optical signal duringtransmission, comprising the steps of: transmitting a wave divisionmultiplexed optical signal having a predetermined set of ranges ofwavelengths; amplifying the wave division multiplexed optical signal togenerate an amplified wave division multiplexed optical signal;monitoring a total optical strength level of at least one of the rangesof the amplified wave division multiplexed optical signal; monitoring aprobe optical strength level of at least one of the wavelengths of theamplified wave division multiplexed optical signal; and adjusting saidamplification based upon the total optical strength level and the probeoptical strength level so as to substantially reduce a gain tilt and anoptical signal-to-noise ratio in the amplified wave division multiplexedoptical signal.
 2. The method of controlling optical signal duringtransmission according to claim 1 wherein said adjusting furthercomprising the steps of: comparing the probe optical strength level to apredetermined gain tilt value to generate a first comparison result; andcontrolling said amplifying step based upon the first comparison result.3. The method of controlling optical signal during transmissionaccording to claim 1 wherein the predetermined gain tilt value isretrieved from a storage table.
 4. The method of controlling opticalsignal during transmission according to claim 1 wherein said adjustingfurther comprising the steps of: comparing the total optical strengthlevel to a predetermined output level value to generate a secondcomparison result; and controlling said amplifying step based upon thesecond comparison result.
 5. The method of controlling optical signalduring transmission according to claim 1 wherein the predeterminedoutput level value is retrieved from a storage table.
 6. The method ofcontrolling optical signal during transmission according to claim 1further comprising additional steps of: monitoring an input totaloptical strength level of at least one of the ranges of the wavedivision multiplexed optical signal; and monitoring an input probeoptical strength level of at least one of the wavelengths of the wavedivision multiplexed optical signal.
 7. The method of controllingoptical signal during transmission according to claim 6 furthercomprising additional steps of: transmitting the amplified wave divisionmultiplexed optical signal to a receiving unit via an optical fiber of apredetermined length; monitoring a transmitted total optical strengthlevel of at least one of the ranges of the amplified wave divisionmultiplexed optical signal at the receiving unit after said transmittingstep; and monitoring a transmitted probe optical strength level of atleast one of the wavelengths of the amplified wave division multiplexedoptical signal at the receiving unit after transmitting step.
 8. Themethod of controlling optical signal during transmission according toclaim 7 wherein a sum of the input total optical strength level at atransmission unit before transmission and the transmitted prove opticalstrength level at a receiving unit after the transmission is identical.9. The method of controlling optical signal during transmissionaccording to claim 7 wherein a sum of the input probe optical strengthlevel at a transmission unit before transmission and the transmittedprobe optical strength level at a receiving unit after the transmissionis identical.
 10. The method of controlling optical signal duringtransmission according to claim 1 further comprising additional stepsof: transmitting the amplified wave division multiplexed optical signalto a receiving unit via an optical fiber of a predetermined length;monitoring a transmitted total optical strength level of at least one ofthe ranges of the amplified wave division multiplexed optical signal atthe receiving unit after said transmitting step; and monitoring atransmitted probe optical strength level of at least one of thewavelengths of the amplified wave division multiplexed optical signal atthe receiving unit after transmitting step.
 11. The method ofcontrolling optical signal during transmission according to claim 10wherein said amplifying step is adjusted based upon the total opticalstrength level, the probe optical strength level, the transmitted totaloptical strength level and the transmitted probe optical strength level.12. The method of controlling optical signal during transmissionaccording to claim 1 wherein said amplifying step is adjusted withrespect to an output level of the amplified wave division multiplexedoptical signal.
 13. The method of controlling optical signal duringtransmission according to claim 1 wherein said amplifying step isadjusted with respect to a gain tilt of the amplified wave divisionmultiplexed optical signal.
 14. A system for controlling optical signalduring transmission, comprising: a first and second optical fibers fortransmitting a wave division multiplexed optical signal having apredetermined set of ranges of wavelengths; an amplifier connected tosaid first optical fiber for amplifying the wave division multiplexedoptical signal according to a predetermined amplification characteristicto generate an amplified wave division multiplexed optical signal, saidamplifier outputting the amplified wave division multiplexed opticalsignal to said second optical fiber; a first monitor connected to saidsecond optical fiber for monitoring a total optical strength level of atleast one of the ranges of the amplified wave division multiplexedoptical signal; a second monitor connected to said second optical fiberfor monitoring a probe optical strength level of at least one of thewavelengths of the amplified wave division multiplexed optical signal;and an adjustment unit connected to said amplifier, said first monitorand said second monitor for adjusting the amplification characteristicbased upon the total optical strength level and the probe opticalstrength level so as to substantially reduce a gain tilt and an opticalsignal-to-noise ratio in the amplified wave division multiplexed opticalsignal.
 15. The system for controlling optical signal duringtransmission according to claim 14 wherein said adjusting unit furthercomprising: a first comparator for comparing the probe optical strengthlevel to a predetermined gain tilt value to generate a first comparisonresult; and a first controlling unit connected to said amplifier andsaid first comparator for controlling the amplification characteristicbased upon the first comparison result.
 16. The system for controllingoptical signal during transmission according to claim 15 furthercomprising a first storage unit connected to said first comparator forstoring the predetermined gain tilt value.
 17. The system forcontrolling optical signal during transmission according to claim 14wherein said adjusting unit further comprising: a second comparator forcomparing the total optical strength level to a predetermined outputlevel value to generate a second comparison result; and a secondcontrolling unit connected to said amplifier and said second comparatorfor controlling said amplification characteristic based upon the secondcomparison result.
 18. The system for controlling optical signal duringtransmission according to claim 17 further comprising a second storageunit connected to said second comparator for storing the predeterminedoutput level value.
 19. The system for controlling optical signal duringtransmission according to claim 14 further comprising: a third monitorconnected to said first optical fiber for monitoring an input totaloptical strength level of at least one of the ranges of the wavedivision multiplexed optical signal; and a fourth monitor connected tosaid first optical for monitoring an input probe optical strength levelof at least one of the wavelengths of the wave division multiplexedoptical signal.
 20. The system for controlling optical signal duringtransmission according to claim 14 further comprising: a receiving unitconnected to said second optical fiber at a predetermined distance fromsaid amplifier for receiving the amplified wave division multiplexedoptical signal as a transmitted wave division multiplexed opticalsignal; a fifth monitor connected to said receiving unit for monitoringa transmitted total optical strength level of one of the ranges of thetransmitted wave division multiplexed optical signal at the receivingunit; and a sixth monitor connected to said receiving unit formonitoring a transmitted probe optical strength level of one of thewavelengths of the transmitted wave division multiplexed optical signal.21. The system for controlling optical signal during transmissionaccording to claim 20 wherein said adjustment unit adjusts saidamplifier based upon a combination of the total optical strength level,the probe optical strength level, the transmitted total optical strengthlevel and the transmitted probe optical strength level.
 22. The systemfor controlling optical signal during transmission according to claim 21wherein said adjustment unit adjusts based upon a sum of the totaloptical strength level of the amplified wave division multiplexedoptical signal at said amplifier before transmission and the transmittedtotal optical strength level at said receiving unit after thetransmission.
 23. The system for controlling optical signal duringtransmission according to claim 21 wherein said adjustment unit adjustsbased upon a sum of the probe optical strength level of the amplifiedwave division multiplexed optical signal at said amplifier beforetransmission and transmitted probe optical strength level at saidreceiving unit after the transmission.
 24. The system for controllingoptical signal during transmission according to claim 14 wherein saidadjustment unit adjusts said amplifier with respect to an output levelof the amplified wave division multiplexed optical signal.
 25. Thesystem for controlling optical signal during transmission according toclaim 14 wherein said adjustment unit adjusts said amplifier withrespect to a gain tilt of the amplified wave division multiplexedoptical signal.